Method of manufacturing porous electrode wire for electric discharge machining and structure of the electrode wire

ABSTRACT

The present invention relates to a porous electrode wire for use in electrical discharge machining and the method of manufacturing the same. The wire improves the machining speed at least 15% compared with a conventional zinc coated wire, which results from an increased cooling ability of the wire with a cooling liquid because of the increase in the surface area of the wire having porous surface morphology. Since the surface of the porous wire presents uniform profile of outer periphery rather than surface protrusions, it does not affect machining accuracy. Further, the porous nature of the wire is expected to improve flushability during the electrical discharge machining, providing spaces to eliminate particles of the machining. Therefore, in accordance with the method of the present invention, a zinc coated wire having improved performance of machining speed and flushability compared with a conventional coated wire can be provided without additional processes.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/kr98/00233 which has an Internationalfiling date of Jul. 30, 1998 which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode wire for use in electricaldischarge machining and the method of manufacturing the same,particularly to a porous electrode wire having an improved machiningspeed and the method of manufacturing the same.

2. Description of the Background Art

FIG. 1 represents a schematic drawing of a wire electrical dischargemachine. An electrode wire is inserted through a start hole(7) of aworkpiece(1), which is continuously fed through the hole. A highfrequency voltage is applied between the wire(2) and the inside of thehole(7) to initiate an arc discharge between them. Then, machining theworkpiece(1) to a desired shape can be achieved by melting the workpieceduring the arc discharge and by removing the machining particles using amachining liquid and an instantaneous vaporization power between thewire and the workpiece. In accordance with the machining principle, thewire electrical discharge machine includes a power supply(6), a wiretransfer means, a workpiece moving means and a circulating means of themachining liquid.

In general, the workpiece moving means, as indicated by the arrow inFIG. 1, moves during the machining of a workpiece on a planeperpendicular to the wire feeding direction. The wire(2) from a supplyspool(3) travels to a take-up roll(4) through a wire transfer meansincluding the upper and the lower guide rollers(5 and 5′) of theworkpiece.

Then, a high frequency voltage is applied between the workpiece(1) andthe electrode wire(2) to start the machining of the workpiece. At thesame time, a machining liquid of deionized water is supplied to themachining area to remove the heat of the machining. The machiningefficiency, in particular the machining speed, significantly depends onmachining parameters such as the feeding speed of the machining liquid,machining current, and the shape and frequency of the machining voltage,and it is known to improve the machining efficiency through a control ofthe machining parameters.

Since copper has a high electrical conductivity and is easy to form finewire due to its high elongation property, a copper wire was usedinitially. However, it revealed many deficiencies mainly due to its lowmechanical strength. For example, high tensile strength could not beapplied to the copper wire during the machining so that vibration of thewire can not be controlled, resulting in an inferior machining accuracyand tendancy of wire breakage. Moreover, machining speed was relativelyslow. Therefore, a molybdemum wire or a tungsten wire as a high strengthwire has been used for a special application of a high precisionmachining. A brass wire having 63-67 wt % copper and 33-37 wt % zinc hasbeen developed for the general purpose of wire electrical dischargemachining.

The brass wire has a tensile strength about twice to a copper wire andthe machining speed can be improved due to the presence of zinc contentin the alloy, which provides a stable discharge and a vaporization powerduring the machining.

Moreover, as the application field of the wire electrical dischargegrows up, it was required for the brass wire to further increase thetensile strength and to improve the machining speed. Therefore, elementssuch as Al and/or Si can be added to a brass wire to improve the tensilestrength and machining speed.

An the other hand, it was known that the machining speed of a brass wireincreases when zinc content includes more than 40 wt % in the brass.However, in that case, drawing process to form a wire becomes difficultbecause of the presence of a brittle phase in the alloy.

U.S. Pat. No. 4,287,404 discloses a zinc coated wire on copper or brasscore and the method of manufacturing the same. On a core material havingrelatively high tensile strength or high electrical conductivity such ascopper, brass or steel, a coating material having a relatively lowvaporization temperature such as zinc, cadmium, tin, antimony, bismuthor the alloy was electroplated to form a coated wire. According to thewire and the method, the core allows to maintain required mechanicalstrength or conductivity, and the coating increases cooling ability andflushability because of its relatively low vaporization temperature,thereby improving machining speed and accuracy. Further, the coatingmaterial vaporizes easily by the heat during the machining, it protectscore material because of the cooling effect of the coating material.Thus, the method of manufacturing the coated wire may include thecoating step of zinc electroplating after the final sizing the wire orprior to the final sizing of the wire.

A method of improving the performance of a coated wire was disclosed inU.S. Pat. No. 4,977,303. According to the patent, the method includessteps of; on a metallic core, a coating step of zinc, cadmium or thealloy which forms mixed alloy layer with the core after heat treatmentby diffusion annealing; a heat treatment step of the coated wire at 700°C. in an oxidizing atmosphere to form a mixed alloy layer between thecore material and the coating material, for example copper-zinc alloyand drawing the coated wire accompanying a mechanical hardening. Thecoated wire by the method includes a core, a mixed alloy layer and anouter oxide layer. At this time, the oxide layer prevents possibility ofshort circuits between the wire and the workpiece during the electricaldischarge machining, which is not directly related to the machiningspeed. The improvement in the machining speed of the wire is known tolie on the heat treatment step forming copper-zinc alloy layer, but themechanism was not clearly revealed.

U.S. Pat. No. 4,686,153 discloses a coated wire having a copper cladsteel core and a coating layer of zinc alloy formed on the core, and themethod of manufacturing the same. The high strength of steel in the corecan provide a superior machining accuracy and the clad copper canprovide a good conductivity to the coated wire. On this copper cladsteel, zinc coating is applied by electroplating or hot dip galvanizingfollowed by heat treatment to form a copper-zinc alloy layer.Particularly, when the zinc content in the alloy layer is in the rangeof 40-50 wt %, the improvement of the coated wire in machining speedbecomes evident compared with a simple zinc coated copper clad steel.The coated wire according the patent includes a copper clad steel coreand a copper-zinc alloy layer. At the same time, the zinc content in thealloy layer ranges 10-50 wt %, preferably 40-50 wt %. The method ofmanufacturing the same includes steps of; a providing step of a copperclad steel core, a zinc electroplating step on the core, a drawing stepof the zinc coated core to form a wire having a desirable diameter and aheat treatment step of the wire to convert the zinc coating layer into acopper zinc alloy layer having zinc content of 10-50 wt %, preferably40-50 wt % in such a manner that the concentration of the zinc isgradually decreased along the radially inward direction. Alternatively,the drawing step may be applied prior to the heat treatment step and thezinc coating may use hot dip galvanizing.

SUMMARY OF THE INVENTION

As mentioned previously, the improvement of the machining speed of thecoated wire was achieved by coating the core with a material such aszinc which have a melting temperature and vaporization temperature lowerthan core material, and a further improvement was achieved by heattreating the zinc layer on the core to form a copper-zinc alloy layerthrough diffusion reaction between the core and the coating. However,the improvement significantly depends on the selection of the coatingmetal having lower vaporization temperature than the core metal. Thus,the improvement was limited to the nature of the coating metal.

A purpose of the present invention is to provide a coated wire forelectrical discharge machining with improved machining speed byincreasing the surface area of the wire which will be in contact withcooling liquid so as to increase the cooling ability of the wire.

Another purpose of the invention is to provide a coated wire forelectrical discharge machining with improved machining speed by allowingthe contact of the cooling liquid not only with the surface of the wirebut also with inner part of the wire.

Still another purpose of the invention is to provide a method ofmanufacturing a porous coated wire with increased surface area withoutadditional steps. Still another purpose of the invention is to provide acoated wire for electrical discharge machining with improvedflushability without decreasing the machining accuracy during themachining.

Therefore, the above mentioned purposes are achieved by the methodincluding the steps of; providing a wire having a first diameter made ofa first metal, hot dip galvanizing the wire by passing the wire in adesirable time through a molten of a second metal having vaporizationtemperature lower than the first metal, thereby forming an alloy layerby the diffusion reaction between the first metal and the second metalhaving hardness higher and an elongation lower than the first metal andthe second metal and a coating layer made of the second metal, anddrawing the wire having the alloy layer and the coating layer to form asecond diameter, thereby forming cracks in the alloy layer and thecoating layer due to the high hardness and the low elongation of thealloy layer.

At this time, the first metal may use copper or brass having 63-67 wt %copper and 33-37 wt % zinc. Further, the second metal may use zinc,aluminum or tin.

Particularly in the present invention, the wire made of the first metalneeds to pass the molten bath in a desirable time so as to achieve adesirable thickness of the coating layer and alloy layer including thesecond metal. The desirable time depends on the length. Foe example, thetake-up speed of the wire should be fast when the length of the bath isrelatively long, and the take-up speed of the wire should be slow whenthe length of the bath is short. Thus, the take-up speed and the lengthof the bath are selected to form the thickness of the coating layerhaving 3-10 μm on the wire having the first diameter.

The method of manufacturing a coated wire according the presentinvention may further include heat treatment step to stabilize themechanical property of the wire.

Further, the method may include removing step of the coating layer onthe alloy layer.

The coated wire for electrical discharge machining, according to thepresent invention, includes a core made of a first metal includingcopper, an alloy layer formed on the core and a coating layer made of asecond metal, wherein the alloy layer having a higher hardness than thecore or the coating layer is formed during the hot dipping galvanizingstep by diffusion reaction between the first metal and the second metalhaving vaporization temperature lower than the first metal, and whereinthe alloy layer includes cracks having direction perpendicular to thelongitudinal direction of the wire.

The alloy layer having a high hardness and a low elongation is formed bydiffusion reaction of the first metal and the second metal during thehot dip galvanizing step passing the wire, having the first diameter,made of the first metal into a molten bath of the second metal havinglower vaporization temperature. Then, the wire coated with the secondmetal is drawn to form the wire having the second diameter. At thistime, the first metal covered by the alloy layer and the coating layerbecomes a core of the wire. Further, the porous nature of the wirearises from the cracks in the alloy layer and the coating layer duringthe drawing step.

Further, the coated wire for electrical discharge machining, accordingto the present invention, includes a core made of a first metalincluding copper and an alloy layer formed on the core, wherein thealloy layer having a higher hardness than the core or the second metalis formed during the hot dipping galvanizing step by diffusion reactionbetween the first metal and the second metal having vaporizationtemperature lower than the first metal, and wherein the alloy layerincludes cracks having direction perpendicular to the longitudinaldirection of the wire.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 is a schematic drawings showing the structure and a principle ofa general wire electrical discharge machining.

FIG. 2 shows steps of manufacturing porous coated wires for electricaldischarge machining.

FIG. 3A is a photograph of a wire after coating step and prior todrawing step according to the present invention.

FIG. 3B is an enlarged view of a part of FIG. 3A.

FIG. 4A is a photograph of a porous coated wire according to the presentinvention, showing the surface morphology.

FIG. 4B is a photograph of a porous coated wire according to the presentinvention, showing the cross-section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in detail with preferredembodiments. FIG. 2 shows the steps of manufacturing porous electrodewires for electrical discharge machining, according to the presentinvention. First, a brass wire having an intermediate diameter of 0.9 mmand made of 35 wt % zinc and 65 wt % copper is provided as a corewire(12). The core wire(12) is passed through a molten bath(10) of zinchaving a vaporization temperature lower than the brass for a given timeto form a coating layer made of zinc on the core wire(12). At this time,an alloy layer is simultaneously formed on the core wire by diffusionreaction of the brass of the core wire and zinc of the coating layer.Thus, the coating layer is formed on the alloy layer which has thehighest hardness and the lowest elongation among others.

With reference to FIG. 3A showing a photograph after the coating step ofzinc on the brass core wire and FIG. 3B showing an enlarged view of FIG.3A, it is possible to observe that the alloy layer(22) and the coatinglayer(23) are formed sequentially on the brass core wire(21).

The coated core wire(12) thus formed is drawn through a drawingmeans(14) to form a desirable diameter of the wire, for example0.05-0.35 mm. Guide rollers (16) and (16′) help guide the wire to thedrawing means (14), and the take-up roll (18) collects the drawn wire.At this time, since the alloy layer between the core and the coatinglayer has the highest hardness and lowest elongation, a porous structureon the surface of the wire is formed during the drawing step due to thecracks in the alloy layer having direction approximately perpendicularto the drawing direction.

The wire is followed by a heat treatment step to stabilize themechanical property.

FIG. 4A shows the surface morphology of the porous wire, and FIG. 4Bshows cross-section of the porous wire according to the presentinvention. With reference to FIG. 4A, it is observed that the crackshave direction approximately perpendicular to the longitudinal directionof the wire. These cracks are formed by drawing the hard alloy layerand, as shown in FIG. 4B, the alloy layer and the coating layer areporous with spongy-like structure.

The structure of the porous coated wire includes a core(21) made ofbrass and the surrounding coating layer(23) made of zinc. The corewire(21) may use copper or brass which includes copper. Thus, the corewire(21) satisfies electrical conductivity and mechanical strengthrequired for the electrode wire for electrical discharge machining andthe coating material(23) protects the core wire(21) and increases themachining speed. At the same time, since the coating layer(23) has aporous structure, the wire reveals improved cooling ability comparedwith a conventional coated wire. This is because the surface area of thewire to contact with cooling liquid significantly increase during theelectrical discharge machining. Since the improved cooling abilityallows to maintain relatively lower temperature for a given material, itis possible for the electrode wire with improved cooling ability toincrease relative electrical conductivity so as to improve the machiningspeed and also possible to improve mechanical strength of the core wireso as to improve the machining accuracy during the machining. Thus, theporous coated wire with improved cooling ability according to thepresent invention is expected to improve the machining accuracy and themachining speed at least 15% faster than a conventional coated wire. Therequirement for the material for the coating includes a meltingtemperature or vaporization temperature lower than that of corematerial, being able to be coated to the core metal including copper orbrass by hot dip galvanizing process and being able to form an alloylayer with the core metal during the coating process by diffusionreaction. These materials include zinc, aluminium and tin. Thus, it ispossible to form a porous structure in the coating layer during thedrawing step of the coated core wire because of the alloy layer betweenthe core and the coating layer having different hardness and elongationproperties than the core.

In FIG. 3A, a relatively uniform distribution of cracks on the coatedwire can be observed. The crack propagation direction is approximatelyperpendicular to the longitudinal direction of the wire which is thedrawing direction of the wire. Further, the cracks are mainly formed byintergranular crack with minor transgranular cracks. On the other hand,The surface of the porous wire maintains uniform profile of peripheryrather than having sharp protrusions. Therefore, The possibility ofevolution of particles from the wire during the machining becomes low.Further, the particles of the machining may be easily removed by thecracks of the porous wire so that the flushability of the wire can beimproved in comparison to the conventional coated wire.

PREFERRED EMBODIMENT 1

A brass wire having 63-67 wt % copper and 33-37 wt % zinc is prepared asa core wire of an intermediate diameter. Hot dip galvanizing ispracticed on the core wire to form a coating layer. The hot dipgalvanizing may use zinc, aluminium, tin or their alloys andparticularly, zinc is preferred. According to a conventional hot dipgalvanizing process, the core wire undergoes pre-treatment of alkalidegrease and acid cleaning. Then, it passes ammonium chloride flux bath.Subsequently, the core wire passes a molten bath of zinc. At this time,the temperature of the bath is maintained 400-500° C. and the core wireis coated for about 1-10 seconds to form a zinc coating layer and acopper-zinc alloy layer. The alloy layer is formed by a diffusionreaction between the core and the zinc and the coating layer of zinc isformed thereon. Further the alloy layer is the hardest layer amongothers and has lower elongation than the core.

Thus, an copper-zinc alloy layer of 1-2 μm and a zinc coating layer 3-8μm are formed on the brass core wire, which are compact layers. Theformation of the alloy layer allows superior adhesion between the corewire and the zinc coating layer.

The coated wire having intermediate diameter is cooled in airatmosphere, then is followed by a drawing step to form a fine wirehaving a desired diameter ranged from 0.05 mm to 0.30 mm. At this time,since the alloy layer has properties of a high hardness and a lowelongation compared with the core wire, the fine wire, during thedrawing step, produces uniform cracks in the coating layer and the alloylayer to form a porous structure on the surface. These cracks arepropagated through the grain boundary of the coating layer and thedirection of the propagation is perpendicular to the longitudinaldirection of the wire since the drawing direction is the same to thelongitudinal direction of the wire. Further, the outer zinc layer may beremoved. Then, the fine wire after the drawing step having a desireddiameter undergoes a heat treatment at 300-600° C. for about 1-2 secondsin order to stabilize its mechanical property, which makes finalproducts of the porous coated wire.

ADVANTAGES OF THE PRESENT INVENTION

The porous coated wire for electrical discharge machining according tothe present invention improves the machining speed about 15% comparedwith a conventional zinc coated wire. This is due to the porousstructure of the wire which increases cooling ability of the wire incontact with a cooling liquid during the machining. On the other hand,since the surface of the porous coated wire maintains uniform profile ofperiphery rather than having sharp protrusions, it does not havenegative effects to the machining accuracy. Rather, the porous natureallows to remove the particles during the machining so as to improvemachining accuracy with improved flushability. Therefore, the methodmanufacturing the porous coated wire according to the present inventiondoes not requires an additional step to improve the machining speed andthe machining accuracy compared with the conventional method ofmanufacturing a coated wire.

What is claimed is:
 1. A method of manufacturing a coated electrode wirefor use in electrical discharge machining comprising: providing anintermediate wire having a first diameter and made of a first metalincluding copper; hot dip galvanizing the intermediate wire through amolten bath of a second metal having vaporization temperature lower thanthe first metal for a desired time and temperature, wherein an alloylayer is formed on the intermediate wire by diffusion reaction of thefirst metal and the second metal, having hardness higher and lowerelongation than the first metal and second metal, and wherein a coatinglayer is formed on the alloy layer; and drawing the intermediate wirehaving the alloy layer and the coating layer to form a coated electrodewire having a second diameter, wherein cracks are formed during thedrawing step in the alloy layer and the coating layer due to the highhardness and low elongation.
 2. The method according to claim 1, furthercomprising: heat treating the coated electrode wire having the seconddiameter to stabilize its mechanical properties.
 3. The method accordingto claim 1, wherein the first metal is made of a brass.
 4. The methodaccording to claim 3, wherein the second metal includes zinc, aluminum,tin or alloys thereof.
 5. The method according to claim 4, wherein thedesired time allows to form the coating layer of 3-10 μm on theintermediate wire.
 6. The method according to claim 1, wherein thesecond metal includes zinc, aluminum, tin or alloys thereof.
 7. Themethod according to claim 6, wherein the desired time allows to form thecoating layer of 3-10 μm on the intermediate wire.
 8. The methodaccording to claim 1, wherein the desired time allows to form thecoating layer of 3-10 μm on the intermediate wire.
 9. The methodaccording to claim 1, further comprising: removing the coating layerformed on the intermediate wire.
 10. The method according to claim 9,wherein the first metal is made of a brass.
 11. The method according toclaim 9, wherein the second metal includes zinc, aluminum, tin or alloysthereof.
 12. The method according to claim 11, wherein the desired timeallows to form the coating layer of 3-10 μm on the intermediate wire.13. The method according to claim 9, wherein the desired time allows toform the coating layer of 3-10 μm on the intermediate wire.
 14. Anelectrode wire for use in electrical discharge machining comprising; acore wire made of a first metal including copper; an alloy layer formedon the core wire; and a coating layer on the alloy layer made of asecond metal having vaporization temperature lower than the first metal,wherein the alloy layer is formed by diffusion reaction between thefirst metal and the second metal and has a higher hardness and lowerelongation than the core wire or the coating layer, and wherein cracksare formed in the alloy layer and the coating layer having directionperpendicular to the longitudinal direction of the electrode wire. 15.The electrode wire according to claim 14, wherein the cracks in thealloy layer and the coating layer is formed during the drwaing of thecore wire in the direction of the longitudinal direction.
 16. Theelectrode wire according to claim 14, wherein the first metal is made ofa brass.
 17. The electrode wire according to claim 16, wherein thesecond metal is made of zinc, aluminum, tin or alloys thereof.
 18. Theelectrode wire according to claim 15, wherein the second metal is madeof zinc, aluminum, tin or alloys thereof.